Aluminum organic electrolytes and method for electrolytic coating with aluminum or aluminum-magnesium-alloys

ABSTRACT

Organoaluminum electrolytes and methods for the coating of electrically conductive materials with aluminum or aluminum-magnesium alloys, essentially and preferably consisting of Na[Et3Al-H-AlEt3] for aluminum coating, or of either K[AlEt4] or Na[Et3Al-H-AlEt3] and Na[AlEt4] and trialkylaluminum for alloy coating using solutions of these electrolytes in liquid aromatic hydrocarbons or mixtures thereof with aliphatic mono- or polybasic ethers, such as dimethoxyethane, and using soluble anodes of aluminum or of aluminum and magnesium, or of aluminum-magnesium alloy.

This application is a 371 of PCT/EP99/09236 filed on Nov. 27, 1999.

The present invention relates to organoaluminum electrolytes suitablefor the electrolytic deposition of aluminum or aluminum-magnesium alloyson electrically conductive materials, and a method for this usingsoluble aluminum anodes or soluble aluminum and magnesium anodes or ananode made of an aluminum-magnesium alloy.

Organoaluminum complex compounds have been used for a long time for theelectrolytic deposition of aluminum (dissertation H. Lehmkuhl, TH Aachen1954, German Patent 1047450; K. Ziegler, H. Lehmkuhl, Z. anorg. allg.Chemie 283 (1956) 414; German Patent 1056377; H. Lehmkuhl, Chem. Ing.Tech. 36 (1964) 616; EP-A-0084816; H. Lehmkuhl, K. Mehler and U. Landauin Adv. in electro-chem. Science and Engineering (Eds. H. Gerischer, C.W. Tobias, Vol. 3, Weinheim 1994). As suitable electrolytes, there havebeen proposed complex compounds of general type MX2AlR₃, which areemployed either as molten salts or in the form of their solutions inliquid aromatic hydrocarbons. MX may be either alkali metal (Na, K, Rb,Cs) or onium halides, preferably fluorides. R represent alkyl residueswith preferably one, two or four carbon atoms.

The interest in electrolytic coatings of metal workpieces with aluminumhas greatly increased due to the excellent corrosion protection by thealuminum layers and their ecological safety. Therefore, the galvaniccoating with organoaluminum electrolytes which work at moderatelyelevated temperatures of between 60 and 150° C. and in closed systems isof great technical importance.

Since it has been sought, in recent years, to develop motor vehiclesoptimized in terms of consumption and weight, a consequent light-weightconstruction more and more requires the use of aluminum or magnesium ortheir mutual alloys. However, the light metal materials have a drawbackin that both aluminum and magnesium have a high solution pressure inaqueous medium. Mainly upon contact with steels or conventionallygalvanized steels, there is contact corrosion. For this reason, it isrequired to coat fixing members on magnesium applications in such a waythat contact corrosion on the magnesium is avoided, on the one hand, anda long-term stability of the coating is obtained, on the other hand. Thegalvanic coating of the connecting screws with aluminum alone servesthis function only partially since the corrosion products of theconstruction material magnesium are alkaline and attack the aluminumsurfaces of the coating (B. Reinhold, S. G. Klose, J. Kopp, Mat.-wiss.u. Werkstofftech. 29, 1-8 (1998).

Methods for the galvanic deposition of aluminum-magnesium alloys onelectrically conducting materials are known: J. H. Connor, W. E. Reedand G. B. Wood, J. Elektrochem. Sc. 104, 38-41 (1957), describe onlybriefly that they obtained a metal layer with 93% Al and 7% Mg having agood appearance upon electrolysis of AlBr₃, Li[AlH₄], MgBr₂ (Mg/Al=0.8)in diethyl ether. J. Eckert and K. Gneupel obtained metal depositionswith up to 13% Mg from a similar electrolyte of AlCl₃, Li[AlH₄], MgBr₂in a mixture of THF, diethyl ether and benzene (Mg/Al=0.6) (GDR PatentSpecification 244573 A1). The conductivity of the electrolyte was on theorder of 1.10⁻³ to 7.10⁻³ S.cm⁻¹. In the GDR Patent Specification 243723A, the same authors describe an electrolyte solution consisting ofethylmagnesium bromide and triethylaluminum in THF/toluene 1:1 fromwhich metal layers with a maximum of 10% Al were obtained.

Typical electrolytes, which have also proven technically useful for thedeposition of aluminum, based on organoaluminum complex compounds of thetype M[R₃Al—X—AlR₃] (R=Et, iso-Bu; X=F, Cl; M=K, Cs, N(CH₃)₄) have beenused for the electrochemical deposition of aluminum-magnesium alloys andmagnesium by A. Mayer, J. Electrochem. Sci. 137 (1990), and in the U.S.Pat. No. 4,778,575 (priority of Oct. 18, 1988) after the addition oftrialkylaluminum (R=Et, i-Bu) and dimethyl- or diethylmagnesium.

However, in a technical application of this method, the followingproblems arise, which render a continuously operating coating processimpossible.

In contrast to aluminum anodes, magnesium anodes cannot be dissolved inthe coating process with the proposed electrolytes. Continuousreplenishing of the Mg content by dissolving the magnesium anode is notpossible using organoaluminum complexes containing fluoride or generallyhalide as electrolytes.

According to the description in the U.S. Pat. No. 4,778,575, dialkylmagnesium in ethereal solution is employed for preparing theelectrolyte. In a continuously working coating method, the dialkylmagnesium would have to be fed constantly in ethereal solution. However,diethyl ether is known to cleave some complexes, e.g., Na[Et₃Al—F—AlEt₃]into Na[Et3AlF]+Et₃Al.OEt₂ (K. Ziegler, R. Köster, H. Lehmkuhl, K.Reinert, Liebigs Ann. Chem. 629, 33-49 (1960)). If the use of ether asthe solvent for dialkyl magnesium was to be avoided, dialkyl magnesiumwould first have to be rendered ether-free, which requires considerableexpenditure and costs, or it would have to be prepared in an ether-freeform by the reaction of magnesium metal with di-alkylmercury, a verytoxic compound.

For the reasons already described, it has been the object of the presentinvention to provide halide-free organoaluminum electrolytes whichcombine in themselves optimally the properties required for a technicalapplication for the deposition of aluminum and aluminum-magnesiumalloys, such as solubility of both aluminum and, in the case of alloylayers, magnesium anodes by electrolysis, as high as possible aconductivity, homogeneous solubility in aromatic solvents, such astoluene at between 20 and 105° C., cathodic deposition of dense layersof aluminum-magnesium alloys with selectable proportions of the twocomponents of from Al:Mg=95:5 to 5:95.

The object has been achieved by the use of organoaluminum electrolyteswhich are characterized by containing either (in the case of electrolytetype I) alkali tetraal-kylaluminate M[AlR₄] or (in the case ofelectrolyte type II) alkali hexaalkylhydrido-dialuminate andadditionally M[AlR₄] as well as trialkylaluminum AlR₃ (R=CH₃, C₂H₅, C₃H₇or n- or iso-C₄H₉; M=Li, Na, K, Rb, Cs), while electrolytes ofcomposition M[R₃Al—H—AlR₃] have proven particularly useful for thepreparation of pure aluminum layers.

For reasons of optimizing solubility, specific conductivity and goodaccessibility, the ethyl compounds (R=C₂H₅=Et) are preferred. Anelectrolyte according to the invention of type I is dissolved in 2.5 to6 mol per mole of complex compound of an aromatic hydrocarbon liquid at20° C., preferably in toluene or a liquid xylene. The trialkylaluminumis preferably triethylaluminum (AlEt₃), and alkali tetraalkyl-aluminateis preferably a mixture of potassium and sodium tetraethylaluminates.The quantitative ratio of complex : AlEt₃ is from 1:0.5 to 1:3,preferably 1:2. The proportion of Na[AlEt₄] is between 0 and 25 molepercent, based on the total amount of K[AlEt₄] and Na[AlEt₄], butpreferably between 5 and 20 mole percent. The addition of low amounts ofNa[AlEt₄] is preferred because, when this component is lacking, thealuminum anodes are dissolved only with moderate to poor currentefficiencies, e.g., only about 22% in K[AlEt₄]/3AlEt₃/6 toluene, whichwould lead to a loss of triethylaluminum for extended durations of theelectrolysis. The electrolysis is performed at temperatures of between80 and 105° C., preferably between 90 and 100° C.

An illustrative electrolyte I is 0.8 mol of K[AlEt₄]/0.2 mol ofNa[AlEt₄]/2.0 mol of AlEt₃/3.3 mol of toluene. From this electrolytesolution, there is no crystallization even upon extended standing atroom temperature, and the specific conductivity at 95° C. is 13.8mS.cm⁻¹.

The addition of at least 0.3-0.5 mol of triethylaluminum is necessary toavoid the deposition of alkali metal during the electrolysis. Theaddition of larger amounts of AlEt₃ (2-3 mol AlEt₃ per mole of complex)has a very positive effect on the alloy deposition; the alloy layersobtained thereby have 5-50% by weight of Mg, are very uniform, have asilky gloss and are essentially pore-free at a layer thickness of as lowas 4-6 μm. However, if the amount of triethylaluminum per mole ofcomplex is increased from 2:1 to 3:1, it is required, in order tomaintain a solution which is homogeneous even at room temperature, toadd additional solvent to the electrolyte, i.e., to a total of 5.5-6 molof toluene per mole of complex. However, the electrolyte losesconductivity thereby.

Electrolytes of type II preferably consist of mixtures ofNa[Et₃Al—H—AlEt₃], Na[AlEt₄] and AlEt₃. Despite of unfavorableproperties of individual components, e.g., a relatively high meltingpoint of Na[AlEt₄] of 125° C. and low solubility in toluene at 20° C.,mixtures of the three components with a suitable mixing ratio (molarratio Na[Et₃Al—H—AlEt₃] to Na[AlEt₄] of between 4:1 and 1:1, preferably2:1) are homogeneously soluble in toluene at 20° C. and then have theproperties required for a technical application for the deposition ofaluminum-magnesium alloy layers, such as the solubility of both aluminumand magnesium anodes by electrolysis, as high as possible aconductivity, homogeneous solubility in aromatic solvents, such astoluene at between 20 and 105° C., cathodic deposition of dense layersof aluminum-magnesium alloys with selectable proportions of the twocomponents of from Al:Mg=95:5 to 5:95. Due to the presence of AlEt₃,aluminum metal is deposited electrolytically from Na[AlEt₄] rather thansodium metal (W. Grimme, dissertation TH Aachen (1960); DBP 1114330(1959); DBP 1146258 (1961)). During electrolysis, Na[AlEt₄] dissolvesboth aluminum and magnesium anodes (W. Grimme, dissertation TH Aachen1960; K. Ziegler, H. Lehmkuhl, in Methoden der Organ. Chem.(Houben-Weyl), Vol. 13, 1, p. 281 (1970).

Electrolytes of composition M[R₃Al—H—AlR₃] (M=Na, K, Rb, Cs; alkylresidue R=CH₃, C₂H₅, C₃H₇, C₄H₉), e.g., Na[Et₃Al—H—AlEt₃], as solutionsin toluene are very highly suitable for the electrolytic deposition anddissolution of aluminum at 90-105° C. However, we have found thatmagnesium anodes are not dissolved in the electrolysis of this compoundin the absence of Na[AlEt₄] according to the invention. After a currentflow of 8.7 mF, the simultaneous use of an aluminum and a magnesiumanode resulted in a weight loss of 8.7 meq of aluminum while themagnesium anode remained completely undissolved. This means thatNa[Et₃Al—H—AlEt₃] without Na[AlEt₄] component represents an excellentelectrolyte for the deposition of pure aluminum. However, for thepreparation of aluminum-magnesium alloy coatings, the combination ofboth Na complexes with triethylaluminum and toluene has the effects

a) that the solubility of NaAlEt₄ is sufficiently increased; and

b) that both aluminum and magnesium anodes are dissolved in thiselectrolysis mixture.

The electrolyte II according to the invention is dissolved in 5-7 molper mole of Na[AlEt₄] of an aromatic hydrocarbon liquid at 20° C.,preferably toluene or a liquid xylene. The quantitative ratio ofNa[Et₃Al—H—AlEt₃] to Na[AlEt₄] is preferably 2:1 to ensure homogeneoussolubility in 6 mol of toluene per mole of Na[AlEt₄], and the molarratio of Na[AlEt₄] to AlEt₃ is preferably 1:2 to ensure perfect metaldeposition by electrolysis. An illustrative electrolyte II is 1 mol ofNa[Et₃Al—H—AlEt₃]/0.5 mol of Na[AlEt₄]/1 mol of AlEt₃/3 mol of toluene.Even upon extended standing at room temperature, there is nocrystallization from this electrolyte solution which would interferewith the technical applicability of the electrolyte. Its specificconductivity at 95° C. is 8.12 mS.cm⁻¹.

The electrolytic deposition of aluminum-magnesium alloy layers from theelectrolytes according to the invention is performed by using a solublealuminum anode and a similarly soluble magnesium anode, or by the use ofan anode made of an aluminum-magnesium alloy. In the case of two anodes,to ensure a continuous operation and for controlling to obtain aselectable and desired alloy composition, the two anodes are separatelyconnected. The electrolyses are performed in toluene solution,conveniently at 90-100° C. The anodic (Al 95-100%; Mg 93-100%) andcathodic current efficiencies are practically quantitative. Since afinite and thus necessary concentration of magnesium in the electrolytebuilds only in the course of the electrolysis, this condition must bebrought about first before a freshly prepared electrolyte is employed.This can be done

1. by a short preliminary electrolysis during which the magnesiumcontent in the cathodically deposited layer increases with increasingmagnesium concentration in the electrolyte solution until the time whena suitable and desired selection of the anodic partial current densitiescauses that as much aluminum and magnesium is anodically dissolved as iscathodically deposited; or

2. by the addition of the complex compound Mg[AlEt₄]₂, a colorlessliquid (K.

Ziegler, E. Holzkamp, Liebigs Ann. Chem. 605, 93-97 (1957)) which mayalso be used as a solution in toluene. After the addition of 0.01 mol ofMg[AlEt₄]₂ per 3.0 mol of K[AlEt₄], for example, electrolyte I can beused directly for the coating according to the inventive method.

The electrolytic deposition from the electrolytes according to theinvention yields aluminum-magnesium alloy layers which are clearlydifferent from previously known layer systems in terms of theirelectrochemical properties. In the cathodic partial reaction, theelectrochemical behavior of the alloy layers corresponds to themagnesium type, and in the anodic partial reaction, it corresponds tothe aluminum type, associated with a pronounced passivity interval.

At room temperature in a 5% aqueous NaCl solution at a pH value of 9.0,the alloy layers have an open-circuit potential of about −1380 to −1500mV versus S.C.E. at Mg incorporation rates of from 5 to 50% by weight.Due to the layer passivity (formation of intermetallic phases), thecathodic partial reaction is additionally inhibited upon contact withmore electronegative metals, such as magnesium. The potential of thecathodic partial reaction is thereby shifted towards even more negativepotential values as compared to the open-circuit potential. As aconsequence, the remaining potential difference between the cathodicpartial reaction of the alloy layer (at pH 9: oxygen reduction) and theanodic partial reaction of the magnesium is highly reduced. Thus, theAlMg alloy layers enable substantial adaptation to the open-circuitpotential of the magnesium alloy AZ91hp, which is about −1680 mV versusS.C.E., and the contact corrosion at the magnesium is highly reduced.Therefore, the alloy layers are suitable for the coating of steel fixingmembers in contact with magnesium. Potential applications include, inparticular, applications in the automobile industry in the gear, engineand car body fields.

In addition, the alloy layers developed, which are deposited fromnon-aqueous electrolytes, are suitable as high quality surface coatingsfor highly heat-treated steel parts whose tensile strength is >1000 MPaand which cannot be coated with conventional galvanic methods due to therisk of hydrogen brittleness. Thus, there is a potential field ofapplications for the coating of heat-treatable and spring steels withalkali-resistant coatings compatible with aluminum and magnesium.

EXAMPLES

Of the subsequent Examples, Examples 1 to 9 relate to electrolyte I,Examples 10 to 14 relate to electrolyte II, and Examples 15 and 16relate to pure aluminum deposition. In Example 17, an Rb[Al(Et)₄]electrolyte was employed.

Example 1

189.5 g (1.14 mol) of Na[AlEt₄] together with 216.8 g (2.35 mol) oftoluene was heated to a bath temperature of 130° C. To the clear andcolorless solution formed in the heat, 85 g (1.14 mol) of dried KCl isadded in small portions. After the addition of the complete amount,stirring is continued for 6 h, the mixture is cooled down to roomtemperature, and the suspension is separated by filtering through aglass fiber thimble, followed by washing with 105 ml (91.0 g; 1.0 mol)of toluene. The total filtrate contains K:Na in a molar ratio of0.79:0.21. Other K:Na ratios, for example, of 0.90:0.10, were adjustedby mixing the pure components K[AlEt₄] and Na[AlEt₄].

Example 2

An electrolyte of composition M[AlEt₄]/3 AlEt₃/6 toluene (M=20 molepercent Na, 80 mole percent K) was electrolyzed at 91-95° C. with arotating round Cu cathode positioned between the Al anode and the Mganode. The current densities were controlled at 0.4 A.dm⁻² for the Alanode and 0.2 A.dm⁻² for the Mg anode, the amount of charge transportedwas 3.5 mF.

After this amount of charge had passed, 2.19 meq of Al and 1.17 meq ofMg had dissolved; the anodic current efficiency was 95.6% based on Aland 96.7% based on Mg. The cathode layer was uniform with a silverygloss and contained 72.4% Al and 27.6% Mg, and the cathodic layerweighed 34.3 mg and was about 12 μm thick.

Upon long-term use of the electrolyte for numerous coating experimentsat 90-95° C., the amount of toluene can gradually decrease byevaporation. When it falls below 5 mol of toluene per mole of M[AlEt₄],the solution becomes inhomogeneous, and some AlEt₃ is segregated in theform of oily droplets. In this case, the amount of toluene must bereplenished to 6 mol of toluene per mole of M[AlEt₄].

Example 3

An electrolyte of composition 0.79 mol K[AlEt₄]/0.21 mol ofNa[AlEt₄]/0.3 mol of AlEt₃/2.5 mol of toluene was electrolyzed at 90-95°C. between Al and Mg anodes and a copper cathode. The cathodic currentdensity was 1 A.dm⁻² and the amount of charge transported was 8.65 mF.Thereafter, 2.77 meq of Al and 4.76 meq of Mg had dissolved, whichcorresponds to an anodic current efficiency of 87%. The cathode layerwas uniform and glossy. It contained 71.0% by weight Al and 29.0% byweight Mg.

Example 4

The electrolyte of Example 3 was again electrolyzed at 90-95° C. afterthe cathode had been replaced by a new copper sheet. The cathodiccurrent density was 0.9 A.dm⁻². After 6.53 mF had passed, the experimentwas discontinued. The cathode layer was uniform and had a silvery gloss.It contained 54.9% by weight Al and 45.1% by weight Mg.

Example 5

The electrolyte of Examples 3 and 4 was electrolyzed four times insuccession using only one magnesium anode. The condition of the cathodelayer and the Al and Mg contents of the electrolyte are represented inTable 1.

TABLE 1 cathode layer content in electrolyte, Experiment % by weightcontent in mat/g No. appearance Al Mg Al Mg 1 uniform, light gray 76.2023.80 2 uniform, getting 53.00 47.00 2.93 0.040 rougher at the edges 3gray, rough at the 29.95 70.05 2.80 0.058 edges 4 gray, rough at the4.60 95.40 2.85 0.070 edges, dendritic

Example 6

An electrolyte of composition K[AlEt₄]/AlEt₃/4 toluene with a specificconductivity of 17.3 mS.cm⁻¹ was electrolyzed at 90-95° C. between ananode consisting of an aluminum sheet and a magnesium sheet and acathode consisting of TiAl₆V₄. The cathodic current density was 0.4A.dm⁻², and after 5.59 mF had passed, 4,53 meq magnesium and 1.02 meq ofaluminum (=99% anodic current efficiency) had dissolved. The cathodelayer was very uniform and silvery and had good adhesion to TiAl₆V₄. Itconsisted of 75% by weight Al and 25% by weight Mg.

Example 7

An electrolyte of composition 0.8 mol of K[AlEt₄]/0.2 mol ofNa[AlEt₄]/2.0 mol of AlEt₃/3.3 mol of toluene was electrolyzed at97-102° C. between 2 anodes consisting of an aluminum-magnesium alloywith 25% by weight Mg and 75% by weight Al and a rotating cylindricalbolt M8 of heat-treatable steel (8.8) with a cathodic current densitywas 0.8 A.dm⁻² and an amount of charge transported of 2.89 mF. Thecathodic and anodic current efficiencies were 99.5% and thusquantitative. The alloy layer of about 9 μm thickness was uniform andhad a silvery gloss and good adhesion to the substrate material.

Example 8

To the electrolyte of Example 7, the bifunctional ether dimethoxyethanewas added with stirring until a ratio of AlEt₃ to DME of 1:0.86 wasreached. After heating to 95-98° C., electrolysis was performed between2 anodes consisting of an aluminum-magnesium alloy with 25% by weight Mgand 75% by weight Al and a rotating cylindrical bolt of 8.8 steel with acathodic current density of 0.8 A.dm⁻² and an amount of chargetransported of 2.99 mF. The anodic current efficiency was 98.8%. Thealloy layer of about 10 μm thickness was very uniform with a mattsilvery appearance and had good adhesion to the substrate material.

Example 9

Example 7 was repeated ten times at 98-100° C., the cathode beingreplaced every time by an uncoated bolt. The respective thicknesses ofthe cathode layer were varied from 9 to 13 μm. Over the ten experiments,the anodic current efficiency was 99.5%.

Example 10

39.8 mol of Na[Et₃Al—H—AlEt₃] and 39.8 mol of Na[AlEt₄] and 78.6 mol oftoluene are stirred at 100° C. Upon cooling to room temperature, finecrystals precipitate from the clear and viscous solution. Addition ofanother 39.8 mol of Na[Et₃Al—H—AlEt₃] and 78.6 mol of toluene andheating to 100° C. yields a clear solution. Its specific conductivity at95° C. is 21.8 mS.cm⁻¹. After the addition of 39.5 mol of toluene, asolution is obtained having a specific conductivity of 19.1 mS.cm⁻¹ at95° C., and upon cooling to room temperature, some more crystalsprecipitate. Therefore, another 39.5 mol of toluene is added, and nocrystallization occurs any more from the solution thus obtained uponcooling. The specific conductivity is 18.0 mS.cm⁻¹ at 95° C. A testelectrolysis between Al and Mg anodes and a steel cathode only yielded agray and rough, partly dendritic cathode layer. To the electrolyte, 79.8mol of AlEt₃ was added, and its specific conductivity was 8.12 mS.cm⁻¹for the thus obtained electrolyte of composition 1 Na[Et₃Al—H—AlEt₃]/0.5Na[AlEt₄]/1 AlEt₃/3 toluene.

Example 11

The electrolyte obtained in the course of Example 10 was electrolyzed at93-98° C. between Al and Mg anodes and a slowly rotating cylindricalcathode of heat-treatable steel (8.8). The anodic current density was0.3 A.dm⁻² for each anode. After 1.6 mF had passed on each anode, theanodic current efficiency was quantitative, and the cathodicallydeposited layer was uniform and had a matt silvery appearance.

Example 12

The electrolyte of Example 11 was electrolyzed at 95-104° C. afterreplacing the cathode by a new one, which was also made ofheat-treatable steel. The anodic current densities were adjusted to 0.45A.dm⁻² for aluminum and 0.15 A.dm⁻² for magnesium. The anodic currentefficiencies were 90%, and the cathode layer was uniform and had asilvery gloss. According to analysis, the layer contained 71.8% Al and28.2% Mg. The layer thickness was 13 μm.

Example 13

After replacing the anodes of Al and Mg by two alloy anodes ofcomposition 75% by weight Al and 25% by weight Mg and after substitutinga new cylindrical cathode of heat-treatable steel 8.8, the electrolyteof Example 12 was electrolyzed at 93° C. During the electrolysis, thecathode rotated slowly between the two anodes. The cathodic currentdensity was 0.8 A.dm⁻². After 3.5 mF had passed, the cathode layer was12 μm thick and uniform and had a matt silvery appearance.

Example 14

After replacing the cathode by an uncoated one, Example 13 was repeatedthree times at 92-100° C. The respective layer thicknesses were variedbetween 10 and 15 μm. Over the 4 experiments, the anodic currentefficiency was 98.9% for the alloy anodes.

Example 15

Al Deposition from Na[Et₃Al—H—AlEt₃]

0.405 mol of Na[(Et₃)AlH] was molten at 90° C., and 0.405 mol of AlEt₃was added. The melt, which was originally somewhat milky, clarified uponthe addition. After cooling to 20° C., a colorless liquid was obtained,which was diluted with 0.81 mol of toluene. The specific conductivity ofthis solution was 22.9 mS.cm⁻¹ at 100° C. A test electrolysis at 90-95°C. between an Al anode and a TiAl₆V₄ at an anodic current density of 0.7A.dm⁻², after 8.7 mF had passed, yielded a silvery, silky cathode layerof aluminum with a quantitative current efficiency. The currentefficiency at the aluminum anode was 96.6%.

Example 16

Al Deposition from K[Et₃Al—H—AlEt₃]

With the mother liquor obtained from the preparation andrecrystallization of K[Et₃Al—H—AlEt₃] (m.p. 138° C.) and having acomposition of 1 mol of K[Et₃Al—H—AlEt₃]/8 toluene with a specificconductivity of 5.2 mS.cm⁻¹, a test electrolysis was performed at 94-96°C. between an Al anode and a Cu cathode at a current density of from 0.6A.dm⁻² to 1.0 A.dm⁻². After 7.73 mF had passed, a uniform silver-graycathode layer was obtained. The current efficiency at the cathode was100.0%, and that at the anode was 99.6%.

Example 17

a) Preparation of Rb[Al(Et)₄]

33.65 g (0.203 mol) of Na[Al(Et)₄] together with 37.3 g (0.405 mol) oftoluene was heated to a bath temperature of 90° C. To the suspension,24.4 g (0.202 mol) of dry RbCl was added in 2 portions. After theaddition, stirring is continued at 90° C. for 14 h. The slightly yellowto orange solution was cooled down to 70° C., and the suspension wasseparated by filtering through a glass fiber thimble, followed bywashing with about 30 g of toluene. The filtrate is a clear solutionwith a slightly rusty color and contains Rb:Na in a molar ratio of0.93:0.07. The analyses correspond to a composition of M[Al(Et)₄] with3.63 toluene (M=Rb+Na). The specific conductivity is 12.9 mS.cm⁻¹ at 95°C.

b) Example of an Rb[Al(Et)₄] Electrolyte with Al(Et)₃ in Toluene

An electrolyte of composition M[Al(Et)₄]/2.17 Al(Et)₃/4 toluene (M=93mole percent Rb, 7 mole percent Na) and a specific conductivity of 8.7mS.cm⁻¹ at 95° C. was electrolyzed at 90-95° C. with a rotating steelbolt (8.8) as the cathode positioned between two AlMg25 alloy anodes.The current density was adjusted to 0.8 A.dm⁻² for the steel cathode.The amount of charge transported was between 3.5 and 6.0 mF in a totalof 6 runs.

At the beginning, the cathode layers were uniform, bright matt, butgradually with a uniform silky silvery gloss, and were between 30 mg and50 mg. The calculated layer thicknesses were from 12 to 20 μm. Theanodic current efficiency was 100% over 6 experiments. The initialcomposition of the layer with a fresh electrolyte was 90.96% Al and9.04% Mg. In the course of the further runs, the system conditioned to alayer composition of 75.02% Al and 24.98% Mg.

c) Example of an Rb[Al(Et)₄] Electrolyte with Al(n-C₃H7)₃ in Toluene

An electrolyte of composition M[Al(Et)₄]/1.98 Al[n-C₃H₇]₃/4.24 toluene(M=93 mole percent Rb, 7 mole percent Na) and a specific conductivity of4.6 mS.cm⁻¹ at 95° C. was electrolyzed at 90-95° C. with a rotatingsteel bolt (8.8) as the cathode positioned between two AlMg25 alloyanodes. The current density was adjusted to from 0.2 to 0.6 A.dm⁻² forthe steel cathode. The amount of charge transported was between 3.5 and7.0 mF. In all current density ranges, the cathode layers were opticallyuneven, dark and matt and were between 27 and 52 mg, and the calculatedlayer thicknesses were between 12 and 23 μm. The anodic currentefficiency was 98.0% over 5 experiments.

What is claimed is:
 1. An electrolyte for the electrolytic deposition ofaluminum-magnesium alloys, characterized by containing an organoaluminummixture essentially consisting of either alkali tetraalkylaluminateM[AlR₄] or alkali hexaalkylhydridodialuminate M[AlR₃—H—AlR₃] and alkalitetraalkylaluminate M[AlR₄]; and trialkylaluminum AlR′₃ and a magnesiumcomponent; wherein M=Li, Na, K, Rb or Cs; and R, R′=CH₃, C₂H₅, C₃H₇ orn- or iso-C₄H₉, wherein R and R′ are the same or different.
 2. Theelectrolyte according to claim 1, wherein said organoaluminum mixture isan ethylorganoaluminum mixture which essentially consists of eitherK[AlEt₄] (A) and Na[AlEt₄] (B) with a molar ratio of B:A within a rangeof 0≦B:A≦1:3; or Na[Et₃Al—H—AlEt₃] (C) and Na[AlEt₄] (D) with a molarratio of D:C within a range of 1:4≦D:C≦1:1; and trialkylaluminum (E). 3.The electrolyte according to claim 2, without an Na[Et₃Al—F—AlEt₃]component, wherein the molar ratio of A:B is between 9:1 and 3:1, andthe molar ratio of (A+B):E is between 1:0.5 and 1:3.
 4. The electrolyteaccording to claim 3, wherein the molar ratio of A:B is 4:1.
 5. Theelectrolyte according to claim 3, wherein said organoaluminum mixture isdissolved in 2-6 mol of toluene, based on the total amount employed ofNa[AlEt₄].
 6. The electrolyte according to claim 2, without a K[AlEt₄]component, wherein the molar ratio of D:C is 1:2 and that of D:E is from1:2 to 1.1.
 7. The electrolyte according to claim 2, and wherein saidorganoaluminum mixture is dissolved in 5-7 mol of toluene, based on theNa[AlEt₄] employed.
 8. The electrolyte according to claim 1, whereintriethylaluminum AlEt₃ is employed as said trialkylaluminum.
 9. Theelectrolyte according to claim 1, wherein said organoaluminum mixture isdissolved in an aromatic hydrocarbon which is liquid at 20° C.
 10. Theelectrolyte according to claim 9, wherein said organoaluminum mixture isdissolved in 2-6 mol of toluene, based on the total amount employed ofNa[AlEt₄] and K[AlEt₄].
 11. The electrolyte according to claim 9,wherein said organoaluminum mixture is dissolved in 5-7 mol of toluene,based on the Na[AlEt₄] employed.
 12. The electrolyte according to claim1, wherein the organoaluminum components are dissolved in a mixture of aliquid aromatic hydrocarbon with an aliphatic mono-, di- or polybasicether R″OR′″ (R″=R′″=alkyl; or R″=alkyl, R′″=CH₂OR″), and the molarratio of AlR₃:R″OR′″ is between 0.5 and 1.0.
 13. The electrolyteaccording to claim 12, wherein the aliphatic ether is dimethoxyethaneCH₃OCH₂CH₂OCH₃, the aromatic hydrocarbon is toluene, and the molar ratioof triethylaluminum:dimethoxyethane is from 0.8 to 0.9.
 14. A method forthe electrolytic deposition of aluminum-magnesium alloys on electricallyconductive materials, wherein an electrolyte according to claim 1 isemployed, and aluminum and magnesium anodes or aluminum-magnesium alloyanodes are used as anodes, the composition of the anode alloycorresponding to that of the desired alloy coating.
 15. The methodaccording to claim 14, which is performed within a temperature range offrom 80 to 105° C.
 16. The method according to claim 14, wherein analloy coating with an aluminum/magnesium ratio of between 95:5 and 5:95is produced.
 17. The method according to claim 14, wherein the magnesiumconcentration in the electrolyte necessary for the sought magnesiumcontent of the alloy coating is adjusted by a preliminary electrolysisor by a single addition of Mg[AlEt₄]₂ at the beginning of theelectrolysis.
 18. The method according to claim 14 for reducing oravoiding contact corrosion on magnesium constructional parts,characterized in that Mg incorporation rates of from 5 to 50% by weightresult in the formation of intermetallic phases within the alloy layer.19. The method according to claim 18, wherein said magnesiumconstructional parts are constructional parts of the automobile industryin the gear, engine and car body fields.
 20. The method according toclaim 14 for avoiding H₂-induced environmental stress cracking, whereinhigh strength steel parts having a tensile strength of >1000 MPa areemployed as electrically conductive materials.
 21. The electrolyteaccording to claim 1, wherein said magnesium component is adjusted tothe desired magnesium concentration in the electrolyte by preliminaryelectrolysis using Al—Mg anodes or by a single addition of Mg[AlR₄]₂.22. A method for electrolytic deposition of aluminum, whereinM[R₃Al—H—AlR₃] is employed as the electrolyte, wherein M=Na, K, Li, Rbor Cs, and the alkyl residue R=C₂H₅, C₃H₇, n- or iso-C₄H₉.
 23. Themethod according to claim 22, wherein M=Na and R=C₂H₅.
 24. The methodaccording to claim 23, which is performed within a temperature range offrom 20 to 105° C.
 25. The method according to claim 24 within atemperature range of between 90 and 100° C.
 26. The method according toclaim 22, wherein the electrolyte is dissolved in a hydrocarbon which isliquid at 20° C.
 27. The method according to claim 26, wherein saidhydrocarbon is toluene.